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  1. #1

    prop slip formula

    I ve got a question on various prop calculators on the net.I have 5000 rpm a 32in bravo1 4 blade and running 81 maybe 82mph.The calculators on the net say that is over 20 % slip.The guys at merc racing tell me its 15 and acceptable ? any comments ? the boat is a 30 deepv with small steps.2001 30y2k superboat.

  2. #2

    just and idea..

    double check the gear ratio. I don't know a whole lot about I/O's but I belive that the upper gear housing can change the ratio some then of coarse the lower unit will change the ratio.
    Lee McCune
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  3. #3
    Thanks Lee ,
    But yes definetely 1.5 ratio ,its a new bravo XR with an imco lower unit . I am new to to these boards and it seems mostly outboard people !(not that thats a bad thing ) well if anyone has any ideas ? anybody with a 30y2k superboat ? id appreciate other set up numbers ,prop pitch ,rpm, mph .
    thanks hey while im asking anybody with experience with hp500efi with a procharger ? iv heard these engines (efi version ) dont take to pro chargers well ?
    thanks again

  4. #4
    It's about 19%.
    At 14% you would be doing 87.
    At 20% is 80.8.
    Since this calculation thing depends on the tach speedo and prop pitch, any one of them can throw an error into it.
    since pitot speedometers lie there is an error.
    The props aren't marked correctly so they can say how "thier 32 is faster or a better hole shot
    Then if the tach is off a little bit then there is another error.

    But none of that makes 20% slip into 15% like you were told, not unless they GPSed it, checked the tach and checked the prop. If you haven't, you can check the speed with a GPS The tach and prop would need something else to check them.

    In affect you have a 25" prop with no slip.
    28" @ 10%
    30" @ 15%
    If you popped on a 28" prop and did get 10% slip you can see the advantage.
    '90 STV
    '96 260
    under construction

    for far too long

  5. #5
    Techno is correct about Tachs and speedos, but most propellers are not marked for their pitch as a marketing ploy. Almost none of the propellers we run now days are straight pitch or straight rake so the location of the center of pressure on the propeller blades determines what pitch and rake the propeller is effectively running at. Props don't slip in some magical invisible way if you have a rig that is running free without excess trim up and not wetting down the sky it is nonsensical to say you have some huge slip percentage. What you have is a combination that results in the center of pressure being located at a point of lesser pitch on the propeller blades.
    except of course N2O

  6. #6
    Originally posted by Nosubforcid
    so the location of the center of pressure on the propeller blades determines what pitch and rake the propeller is effectively running at. .

    I have to disagree with that. The prop pitch is a definate angle of the blade. True this angle can be a variable number from some inches to so many inches. But whatever the propeffectively runs at is prop slip. What it would theoreticaly run at is the stamped pitch. The stamped pitch is supposed to be universally a standard measurement. Even the blades are not the same pitch to each other but the stamped number is supposed to be a comparable measurement.

    If the hull was dirty or messed up in someway or the prop was getting turbulent water the slip could still be high but be running free and with good trim. If everything is tuned like you said then you would just live with the slip % or get a different prop type.

    The marketing ploy thing was mentioned by someone else who seemed to know. But considering what these props cost they could at least sell them with the same pitch on all the blades.
    '90 STV
    '96 260
    under construction

    for far too long

  7. #7
    I have a hard time also believing that Merc mis labels their blades as a marketing ploy. I think Techno hit the nail on the head with his first post........check the tach and speedo.

    My sterndrive boat is a 22' Pachanga with an HP 575 Mercruiser (540 C.I, 575 HP) I swap back and forth between a Bravo 1 and a Blackhawk drive. I've never seen any slip numbers like the ones you're posting on this boat and it has no setback and no notch in the bottom. If anything, the 4 blade prop should make your slip numbers less. You mentioned Imco, is your lower one of their short units? If so, you may be slippin' it that much, but otherwise I think either your blade is mislabeled or your tach/speedo combination is wrong.



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  8. #8
    Nosubforcid
    Just a few questions. Since the blade is marked as a 28" did it make sense that it was stamped this? I figure you measured it so have an idea.
    Did you measure all the blades? Wondering if they were close or the same.


    Why would/does the C.P. change? I don't understand that. It seems that it would be the same unless it's something like the attack angle of a wing being like the trim angle of the shaft?

    What I think the blade should have been stamped is what someone posted before, I've forgotten but it had to do with a certain distance of the prop Percentage of the radius and a distance from the leading edge???

    While I think the method you mentioned to be more accurate and useful the STV still slipped at 15%. Or a different way of saying it is this was a lousy prop for the boat since it is throwing away all the pitch above the 26" but is still spinning a larger prop. If it had been stamped at 30"(the Allison prover) the slip would be a whopping 20%! By saying the slip is 9% on the 26" section is ignoring the pitch above this. I personaly would think it slipped too high. What does running 23"-31" provide over like 23" -26"?
    Do you know how a different 26" performed? That would be proof of the puddin.

    I'll have to search for the other post that mentions the marketing ploy and stamping convention. Sometimes brain cells trade places.

    Are those pitch gauges still 'spensive? Where do I get one?
    '90 STV
    '96 260
    under construction

    for far too long

  9. #9
    Couldn't find it, Oh well. Maybe whoever it was will re-say it.
    '90 STV
    '96 260
    under construction

    for far too long

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